I. Field of the Invention
The present invention relates generally to the field of ground fault relays (GFRs) and, more particularly, to GFR devices in which a sensitive relay detects insulation degradation in large electric motors and cables supplying large electric motors. In current practice, the GFR typically reads the current leakage to ground when the motor is turned on and during running of the motor. If the leaking current is above some preset value, the GFR senses this situation and its output, which is normally connected to the motor controller, then causes the motor to be switched off. The present invention operates in two (2) modes. The first mode is a "look ahead" mode prior to the motor being turned on, and the second mode is a "real time" mode after the motor is turned on. In both modes, the GFR evaluates the insulation resistance to ground.
II. Description of the Related Art
Motor control circuits for protecting an electric motor against ground faults are well known in the prior art, when used in both single phase motors and in three phase motors. Typically, three phase motors draw large currents when running and this is especially true in mining operations. However, to be effective, a GFR, in such an environment as a mining, must sense very small amounts of leakage current.
For instance, in U.S. Pat. No. 5,386,183 issued to Cronvich et al., a ground fault relay apparatus for sensing a ground fault in a motor control system is shown wherein the system generates an unrectified signal indicating the current level in two (2) or more conductors of a motor, and then generates from the first and second unrectified signals an unrectified sum signal indicative of the sum of the current levels and then has means for comparing the rectified sum signal to a designated limit to compare that signal with a preset signal to turn off a motor in which the ground fault currents are excessively large.
In U.S. Pat. No. 5,715,125 issued to Neiger et al., an intelligent ground fault circuit relay is shown wherein the relay automatically tests its internal circuitry on a periodic basis to provide a high probability of proper operation in the event of a ground fault in the motor circuit. In addition, this invention discloses a method of monitoring the ground leakage current and to adjust the trip threshold of the GFR to prevent nuisance tripping. Further, this particular patent discloses systems for monitoring the wiring system for detecting a miswired condition and to set various alarms to indicate the various monitored results to a user.